1. Field of the Invention
The present invention relates to a distributed control system including a plurality of PLCs (programmable controller) connected with one another.
2. Description of the Related Art
Conventionally, in order to control machine facilities at a plurality of locations in factory automation, there has been widely used a distributed control system in which a plurality of PLCs for controlling the respective machine facilities are connected with one another via a transmission line (see, for example, Japanese Patent Application Laid-Open (kokai No. 2002-358106).
Such a conventional distributed control system is configured to transmit a diagnosis command to each PLC via the transmission line as a part of initial operation performed after, for example, the power supply is turned on, and to determine whether or not the system is anomalous on the basis of the results of the diagnosis. When the system is determined to be anomalous, the system is stopped temporarily, and an operator takes necessary measures to recovery the system.
However, in the conventional distributed control system, a diagnosis command is merely transmitted through a single transmission line in its transmission direction. Therefore, the conventional system has a drawback in that the location of an anomaly cannot be specified although determination as to whether or not an anomaly has occurred can be performed. Therefore, when the system is determined to be anomalous, the operator must thoroughly check the entire system, and a very long time is required to recover the system.